1. Field of Invention
The invention relates to a method for adjusting the print repeat length of a print image in a multicolor rotary printing machine.
2. Description of the Prior Art
In a multicolor rotary printing machine, the multicolor print image is transmitted to the print substrate by multiple printing cylinders—the so-called plate cylinders—each transferring a color to the print substrate one after the other, wherein the regions of the plate cylinder which carry the color ink entirely or partially transfer this color ink to the print substrate. Multiple colors which overlap then form the print image.
The print substrate in this case is guided via one or multiple central impression cylinders. If there is only one central impression cylinder, the plate cylinder is positioned against this central impression cylinder, such that the color inks are transferred as completely as possible.
If there are multiple central impression cylinders, a single plate cylinder is frequently functionally assigned to each central impression cylinder such that the peripheral velocity can be adjusted very precisely. However, two or even more plate cylinders can also be functionally assigned to each central impression cylinder.
In general, the peripheral velocities of all plate cylinders should be identical, and also match the peripheral velocity or velocities of the central impression cylinder or central impression cylinders. In the printing machines used today, each individual cylinder is equipped with its own, position-controlled drive such that the peripheral velocity can be very precisely adjusted. High print quality is achieved when all rolls participating in the printing process have the same peripheral velocities.
For a given circumference of the plate cylinders, which can also be identical for each of the same, the so-called print repeat length is defined by one rolling of the plate cylinder on the print substrate. If the print repeat length is changed, the plate cylinder or parts thereof—particularly the so-called print sleeve—must normally be exchanged for a larger or smaller circumference.
However, U.S. Pat. No. 7,584,699 B2 suggests modifying the peripheral velocity of the plate cylinder relative to the peripheral velocity of the central impression cylinder or the central impression cylinders, in order to thereby make it possible to vary the print repeat length. The assumption is made that a modification of less than 1% of the original print repeat length has no effect, or at least no noticeable effect, on the print quality. The background to this suggestion is that the printed and re-rolled print substrate, consisting of plastic, changes its length over time. As such, relaxation effects can arise which shrink the film or stretch the same. In order to then make it possible to observe a print image of the desired length, the shrinkage or elongation is anticipated, and an accordingly elongated or shortened print image is produced during the print process, and the plate cylinder is not exchanged for another.
Even though modern printing machines have a position-controlled drive for each cylinder, it is only possible to carry out such a modified adjustment of the peripheral velocity when the printing machine is halted. This consumes a great deal of time, and is therefore costly.